During the production of olefin polymers in a commercial reactor it will often be necessary to transition from one type of catalyst system producing polymers having certain properties and characteristics to another catalyst system producing polymers having different specifications. The transition between similar or compatible catalysts generally takes place easily. However, when the catalysts are of different types and/or incompatible the process is typically complicated. For example, to change from a traditional chromium-based catalyst to a metallocene catalyst, or vice versa, normally requires a long transition period. Moreover, the polyolefins produced during this transition period will continuously change in properties. If the transition from one catalyst system to another requires substantial changes in reactor conditions, the risks of encountering production problems and producing polymers having extreme properties are likely.
A transition from a polymerization reaction catalyzed by a first catalyst to a polymerization reaction catalyzed by a second catalyst is usually performed by stopping the polymerization process, empty the reactor, recharge and then introduce the second catalyst into the reactor. Such catalyst changes are time-consuming and costly because a reactor shut-down for an extended period of time is necessary during transition.
However, a polymerization reaction may be inhibited or stopped temporarily or permanently without emptying the reactor in a number of ways.
NO 178 152 discloses a method for deactivating a chromium catalyst which also contains an organometallic compound used in the gas phase polymerization of olefins. A group of suitable deactivating agents comprises oxygen, ammonium, water and carbonmonoxide. The polymerization reaction may be restarted without emptying the reactor by first feeding an organoaluminium compound and subsequently the catalyst.
EP 0 604 993 discloses a method for the restarting of a vapour phase olefin polymerization that has been temporarily shut down. The olefin polymerization, utilizing a Ziegler catalyst, is stopped by introducing a deactivator, such as oxygen, water, carbondioxide, carbonmonoxide, alcohols or ketones. The reaction system is restarted by feeding an organoaluminium compound into the reaction system without discharging the previously formed polymer particles followed by the solid catalyst component.
WO 92/14766 describes the use of one volatile and one non-volatile catalyst killer for metallocene catalysts in a high pressure polymerization process, which is added downstream of the polymerization zone to suppress polymerization of the recycling monomer and the separated molten polymer. For metallocene/aluminoxane-based catalyst systems there may also be used methanol and n-butanol as catalyst killers.
EP 0 531 834 describes a method of preparing a Ziegler-Natta catalyst system that is fed to a prepolymerization reactor where .alpha.-olefins, in particular propylene, are prepolymerized. The polymerization is interrupted by introducing gaseous CO.sub.2 directly into the suspension or into the reactor head space for 0.5 to 3 h while stirring the suspension. The obtained catalyst system may be stored as a solid and prior to use reactivated by reacting with a cocatalyst, such as triethyl-aluminium, and an electron donor compound. EP 0 558 987 discloses the production of propylene-ethylene copolymers by using this catalyst system.
U.S. Pat. No. 4,460,755 discloses a method of transitioning a continuous olefin polymerization reaction catalyzed by a Ziegler type catalyst into one catalyzed by a chromium-based catalyst, without the need of emptying and recharging the polymerization reactor. The procedure comprises the steps of stopping the catalyst feed, introducing a hydroxyl-containing silica which reacts with the catalyst, and finally introducing a chromium-based catalyst, while maintaining polymerization conditions during the transition.
WO 95/26370 discloses a process for transitioning from a polymerization reaction catalyzed by a Ziegler-Natta catalyst to a metallocene catalyst. This is accomplished by a) discontinuing the feeding of the first catalyst into the reactor, b) introducing a reversible catalyst killer, c) introducing an irreversible catalyst killer, and d) feeding the second catalyst into the reactor. The Ziegler-Natta catalyst comprises silica impregnated with titanium chloride, magnesium chloride and tetrahydrofuran, and organoaluminium compounds. The metallocene catalyst comprises silica mixed with methylaluminoxane and bis-n-butylcyclopentadienyl-zirconiumdichloride and triethylaluminium as a co-catalyst. Only the transition from Ziegler-Natta catalysts to metallocene catalysts are exemplified. Carbonmonoxide (CO) is used as a reversible catalyst killer and water as an irreversible catalyst killer.
In olefin polymerizations chromium oxide-based catalysts are frequently used. Recent catalyst developments have resulted in metallocene catalysts which comprise metallocene compounds of transition metals. These two types of catalysts are presently of great economic importance and therefore a time-saving transition from one of said catalysts to the other in the same polymerization plant would be highly desireable. However, said catalysts are incompatible and a direct transition between them normally is difficult.